S&P, Moody's and Fitch have now all placed Russia just one notch above junk status.

Russia's state debts may be minimal but its private companies and banks are due to repay around $100bn of its debts in 2015, according to Reuters.

"We note that S&P's rating for Russia is currently at the low boundary of the investment-grade category and would lose such a status if downgraded," analysts at Sberbank Investment Research wrote in a note.

"Things would become more complicated if two of three ratings are moved to speculative."